1. The Field of the Invention
This application relates generally to drilling methods and devices used in drilling. In particular, this application relates to a method and apparatus for slough removal from a borehole that is created during drilling.
2. The Relevant Technology
Many drilling processes are currently known and used. One type of drilling process, exploration drilling, often includes retrieving a sample of a desired material from below the surface of the earth. In a conventional process used in exploration drilling, an open-faced drill bit is attached to the bottom or leading edge of a core barrel for retrieving the desired sample. The core barrel is then attached to a drill string, which is a series of connected drill rods that are assembled section by section as the core barrel moves deeper into the formation. The core barrel is rotated and/or pushed into the desired sub-surface formation to obtain a sample of the desired material (often called a core sample). Once the sample is obtained, the core barrel containing the core sample is retrieved by removing (or tripping out) the entire drill string out of the hole that has been drilled (the borehole). Each section of the drill rod must be sequentially removed from the borehole. The core sample can then be removed from the core barrel.
An outer casing with a larger diameter than the core barrel can be used to maintain an open borehole. Like the core barrel, the casing contains an open-faced drill bit that is connected to a drill string, but both with a wider diameter than the core barrel (and the drill string used with the core barrel). The outer casing is advanced and removed in the same manner as the core barrel, i.e., by tripping the sections of the drill rod in and out. Typically, though, where the casing is used in conventional exploration drilling, it creates a borehole first through which the core barrel (along with its drill string) is used.
In a wireline exploration drilling process, however, the core barrel and the casing are advanced together into the formation. The casing again has a drill bit connected to a drill string and is advanced into the formation. But the core barrel does not contain a drill bit and is not connected to a drill string. Instead, the core barrel rests just inside and on the casing and advances into the formation along with the casing. When the core sample is obtained, the core barrel is retrieved using a wireline system as known in the art, the core sample is removed, and the core barrel is dropped back into the casing using the wireline system. Thus, the wireline system removes the time needed to trip the drill rods in and out of the borehole when obtaining a core sample.
In sonic drilling processes used in exploration drilling, whether conventional or wireline, variable frequency vibration is created by an oscillator that is mechanically induced to the drill string of the core barrel and/or casing. The vibration is transmitted in an axial direction down through the drill string to the open-faced drill bit. The drill string may optionally be rotated and mechanically pushed as it is vibrated into the sub-surface formation.
These drilling processes can have several drawbacks. One of these drawbacks is slough creation. Slough is that material that is displaced by the drill bit, or other material that falls into the borehole. Excess slough will take the path of the least resistance during drilling processes. As a result, the slough will enter the core barrel which can cause disturbed, elongated, compacted, and in some cases, heated core samples. The slough can also be pushed outward into the formation, causing compaction of the formation and alter its natural state, which in turn can cause contamination of the core sample with material that does not belong to the depth of the formation being tested. Additionally, the slough can also enter the annular space between the outer casing and the borehole wall, resulting in increased friction and heat that may cause the casing to bind and become stuck in the borehole.